It is widely recognized that fossil fuels, such as petroleum and charcoal, will be consumed soon. In addition, the by-products generated from fossil fuels can contribute to global warming problems due to greenhouse gases such as carbon dioxide. The planet capability of natural purification has been decreased, and that is detrimental to the survival of humans. Thus, there are world-wide efforts to develop environmentally friendly energy resources and devices with low energy consumption and/or driven by renewable energy such as solar energy. Solar cells or photovoltaic (PV) cells are a type of optoelectronic device which generate electricity through the absorption of electromagnetic radiation (UV, visible, and IR radiation). To date, photovoltaic cells made from inorganic semiconductors, such as Si, CdTe, and CuInxGa1-xSe, have dominated the solar cell industry. Particularly, poly-silicon (poly-Si) and single crystal silicon (SC-Si) together contributed to nearly 90% of the market share. However, the shortage of Si materials, and the high processing cost of inorganic semiconductors have posted a drawback in the development of the solar cell industry.
In contrast, the potential low cost, ease of process in large scale production, and compatibility on flexible substrates of organic semiconductors are appealing for photovoltaic applications. Organic photovoltaic (OPV) cells comprise at least one component that utilize organic or organometallic small molecules or polymeric materials for light absorption and charge processes. Harnessing the power of chemical synthesis, a large variety of organic molecules or polymers with different band gaps and absorption coefficients can be synthesized to maximize the light absorption and power generated from the photovoltaic cells. Thus, the organic photovoltaic cell has emerged as a new class of solar cell technologies. (Prog. Photovolt: Res. Appl. 2007; 15, 659).
Within the area of organic photovoltaic cells, various device architectures have been explored including the dye-sensitized solar cell (DSSC), organic/inorganic hybrid organic cells, and organic photovoltaic cells with heterojunctions. In 1986, C. W. Tang found that a bilayer heterojunction structure fabricated from copper phthalocyanine (CuPc) and perylene tetracarboxylic derivative gives a power conversion efficiency (PCE) of 0.95% at Eastman Kodak. Other small molecules such as pentacene, tetracene, and metal phthalocyanines (PCs) are among the most studied donor materials. (Appl. Phys. Lett., 2004, 85, 5427; Appl. Phys. Lett., 2004, 85, 6272; Appl., Phys. Lett., 2005, 86, 243506) Notably, a combination of fullerene (C60) or perylene and organic semiconductor, which has been under development for electronic materials applications since the 1990's, is the most popular structure of organic solar cells. Starting from the 2000's, a PCE of more than 6% has been achieved (Principles of Solar Cells, Hongreung Publication).
Recently, tremendous progress has been made in the development of OPV cells based on π-conjugated semiconductors as electron donor (p-type) materials, such as regio-regular poly(3-alkylthiophene)s (P3HTs), (Adv. Mater. 2006, 18, 572; Appl. Phys. Lett. 2005, 87, 083506; Nat. Mater. 2006, 5, 197) platinum(II) polyyne polymer, (Nature Mater. 2007, 6, 521), and oligothiophenes (Adv. Mater. 2006, 18, 2872). Devices with greater than 5% PCE have been achieved from nanostructures of an interpenetrating donor/acceptor polymer network, which is prepared by limiting the solvent evaporation rate or post-fabrication thermal annealing. (Adv. Funct. Mater. 2003, 13, 85; Appl. Phys. Lett. 2005, 86, 063502; Adv. Funct. Mater. 2005, 15, 1617; Nat. Mater. 2005, 4, 864)
Bis-(8-quinolinolato-N,O)platinum (II) (PtQ2) complexes were first prepared by R. Ballardini et. al. in 1978 (Inorganica Chimica Acta 1978, 31, 1, L423-L424), which show orange to red emissions in dilute solutions. However, no practical application for PtQ2 complexes was found until 2008. In 2008, Che fabricated deep red to near infrared organic light-emitting diodes (OLEDs) from PtQ2 complexes (Applied Physics Letter 2008, 92, 16, 163305). As the device efficiency is low (up to 0.32 cd/A), no further development in OLED application has been made.
The references cited in this application are incorporated herein by reference.